The present invention relates to a fluid pressure control for smoothly effecting shifting of a transmission of a vehicle, which is a general term for automobiles, industrial vehicles, vehicles for use in a building site and the others.
It more specifically relates to a fluid pressure control which is so constructed as to, in a transmission provided with one or more speed-change clutches, control a fill pressure to the lowest necessary value in order to ensure smooth engagement of the clutches when the fill pressure is flowed into clutch cylinders concerned in response to speed change to regulate a pressure rise from the pressure upon completion of the fill up to the final pressure of engagement and to maintain the final pressure of engagement thus attained.
Conventionally, there has been proposed a combination of a regulator valve, an accumulator and an orifice in order to attain the above-mentioned functions. However, in the above-mentioned combination, it is complicated and difficult to balance the orifice and flow path resistance, the clutch cylinder volume and the accumulator volume in order to obtain the lowest pressure required. In addition, it is liable to be affected by temperature change during the balancing operation.
An improved technique thereof is disclosed, for example, in Japanese patent publication No. 46-21137 in which there are provided a pressure modulating safety valve 28 and a differential pressure valve 29, the pressure modulating safety valve being discharged by the differential pressure valve upon effecting a speed shift such that the pressure modulating safety valve can perform the following modulation of the main hydraulic fluid.
In addition, U.S. Pat. Nos. 4,132,302, 4,216,851 (corresponding to Japanese patent publication No. 61-60298) and 4,311,068 disclose an invention in which a rate of rise valve assembly 108 including a pressure modulating valve 58 and a load piston 164 and a signal pressure control assembly 176 including a dump valve 174 and a clutch fill piston 194 are provided in a common bore 34 so as to form a signal pressure chamber 82 there between, and upon effecting a speed shift, the dump valve is actuated by a difference in pressure between a differential pressure chamber 80 within the signal pressure control assembly and the signal pressure chamber to discharge the rate of rise valve such that the rate of rise valve can perform the following modulation of fluid pressure.
However, in the above-mentioned prior art, the structure is complicated due to the provision of the dump valve (differential pressure valve) which is separately provided to discharge the rate of rise valve (the fluid pressure control) and there is a possibility that a lag is caused during the operation. In addition, the accurate operation can not be ensured due to the provision of the separate dump valve and is suspectible to the intrusion of dust and dirt or the difference in machining precision.